[PATCH v2 19/25] netfs: Speed up buffered reading

[Manu Bretelle <chantr4@gmail.com>]

Hi David,


It seems this commit (ee4cdf7ba857: "netfs: Speed up buffered reading") broke
booting vms using qemu. It still reproduces on top of linux-master.

BPF CI has failed to boot kernels with the following trace [0]. Bisect narrowed
it down to this commit.
Reverting ee4cdf7ba857 on to of current bpf-next master with [1] (basically
ee4cdf7ba857 where I had to manually edit some conflict to the best of my
uneducated knowledge) gets qemu boot back on track.

This can be reproed by following the build steps in [2]. Assuming danobi/vmtest
[3] is already installed, here is the script used during bisect.

  #!/bin/bash
  cat tools/testing/selftests/bpf/config{,.$(uname -m),.vm} > .config
  make olddefconfig
  make -j$((4* $(nproc))) || exit 125
  timeout 10 vmtest -k $(make -s image_name) "echo yeah"
  exit $?

The qemu command invoked by vmtest is:

qemu-system-x86_64 "-nodefaults" "-display" "none" "-serial" "mon:stdio" \
  "-enable-kvm" "-cpu" "host" "-qmp" "unix:/tmp/qmp-971717.sock,server=on,wait=off" \
  "-chardev" "socket,path=/tmp/qga-888301.sock,server=on,wait=off,id=qga0" \
  "-device" "virtio-serial" \
  "-device" "virtserialport,chardev=qga0,name=org.qemu.guest_agent.0" \
  "--device" "virtio-serial" \
  "-chardev" "socket,path=/tmp/cmdout-508724.sock,server=on,wait=off,id=cmdout" \
  "--device" "virtserialport,chardev=cmdout,name=org.qemu.virtio_serial.0" \
  "-virtfs" "local,id=root,path=/,mount_tag=/dev/root,security_model=none,multidevs=remap" \
  "-kernel" "/data/users/chantra/linux/arch/x86/boot/bzImage" \
  "-no-reboot" "-append" "rootfstype=9p rootflags=trans=virtio,cache=mmap,msize=1048576 rw earlyprintk=serial,0,115200 printk.devkmsg=on console=0,115200 loglevel=7 raid=noautodetect init=/tmp/vmtest-init4PdCA.sh panic=-1" \
  "-virtfs" "local,id=shared,path=/data/users/chantra/linux,mount_tag=vmtest-shared,security_model=none,multidevs=remap" \
  "-smp" "2" "-m" "4G"


[0] https://gist.github.com/chantra/683d9d085c28b7971bbc6f76652c22f3
[1] https://gist.github.com/chantra/642868407d10626fd44febdfed0a4fce
[2] https://chantra.github.io/bpfcitools/bpf-local-development.html#building-a-vm-friendly-kernel-for-bpf
[3] https://github.com/danobi/vmtest